This invention relates to hydrophilic polymer membranes which comprise polyvinyl alcohol and chitosan and are excellent in permeability and mechanical strength. These polymer membranes are advantageously used as ultrafilter membranes and semi-permeable (dialytic) membranes, in particular, those for use in artificial kidneys.
Hitherto, hydrophilic polymer membranes comprised of cellulose or polyvinly alcohol are known. Such hydrophilic polymer membranes are used as semi-permeable membranes or ultrafilter membranes for separating substances in a system where water is present. Membranes for these uses are required to be excellent in solute permeability and mechanical strength in the wet state. A number of studies have been made with respect to natural and synthetic polymer materials for use as dialysis membranes and as a result, cellulosic membranes are found relatively good in both permeability and mechanical strength, though not entirely satisfactory. For practical applications, however, further improvement in both properties is required.
Polyvinyl alcohol membranes have very excellent permeability in water but are extremely poor in mechanical strength. Although much research has have been done for the purpose of enhancing the mechanical strength, appreciable improvement has not yet been attained. Chitosan, which is obtained by deacetylation of chitin, widely distributed in the natural world, is better in permeability than cellulosic membranes but, it is poorer in mechanical strength than cellulosic membranes. Thus, chitosan cannot be said to be superior to cellulosic membranes.
In general, improvement of a membranes in both permeability and mechanical strength has been deemed extremely difficult in view of the fact that the permeation rate tends to increase in proportion to the degree of water absorption of a membrane, while contrarily the mechanical strength easily tends to decrease with increase of the degree of water absorption. For instance, polyvinyl alcohol membranes become water-resistant by a thermal treatment, but the membranes thus treated are very poor in mechanical strength in the wet state so that they can hardly be applied to practical uses. The mechanical strength of polyvinyl alcohol membranes can be enhanced by acetalization or the like reaction. However, the degree of water absorption and permeability characteristics are considerably lessened along with the increase of mechanical strength. In the prior art, therefore, it is extremely difficult to obtain polymer membranes which are satisfactory in permeability and mechanical strength. Accordingly, there is a great demand for development of a polymer membrane furnished with excellent permeability and mechanical strength.